Recent development and distribution of mobile tools and electric motors have led into increased demand for high-capacity energy sources such as lithium secondary batteries, for representative example. At present, carbon material such as graphite, hard carbon, etc. is used for the negative electrode active material of the lithium secondary battery.
For example, graphite has a theoretical capacity of 372 mAh/g defined by the first stage structure C6Li formed by intercalation reaction, and it has been used in increasing amount to achieve higher capacity of the battery so far, but now reached the limit. Further, hard carbon may be used and may achieve capacity exceeding the theoretical capacity of graphite, but it can hardly realize high-capacity secondary battery, considering factors such as low initial efficiency, low electrode density, and so on. Considering the above, use of silicon for a negative electrode active material has been proposed, since silicon, as a novel material that can replace the carbon material, exhibits theoretical capacity as high as 4200 mAh/g by alloying with lithium.
Japanese Unexamined Patent Application Publication No. Hei 6-325765 (Patent Document 1) proposes lithium ion intercalating and de-intercalating material consisting of Li-containing silicon oxide or silicate. Use of silicon oxide mixed in a small amount with graphite has been commercialized, since silicon oxide having the coexistence of amorphous or microcrystalline silicon phase and silicon oxide in single particles can provide relatively superior charge-discharge cycle characteristics. However, in practice, considerably low initial efficiency hinders its use in an increased mixing amount. Further, when a material containing high-capacity silicon is used as a negative electrode active material, expansion/contraction of silicon (phase) during charging and discharging can be accompanied with pulverization, which leads into formation of insufficient conducting paths in the electrodes and subsequently, rapid capacity fading as well as deterioration of charge-discharge cycle characteristics could occur.
In order to address the issues related with the use of high-capacity silicon-based materials as the negative electrode active material, WO2004/109839 (Patent Document 2) proposes a negative electrode with amorphous silicon thin film or amorphous thin film containing silicon as a main component, directly deposited on a current collector by sputtering, and so on. According to the constitution of Patent Document 2, it appears possible to reduce thickness increase of an active material layer after charging and discharging, since it is possible to provide superior charging and discharging capacity and cycle characteristics, and suppress porosification of the active material layer due to charging and discharging.
Additionally, Japanese Unexamined Patent Application Publication No. 2001-297766 (Patent Document 3) discloses that, by employing alloy particles of silicon phase including silicon with Li intercalating capability and metal phase without Li intercalating capability, it is possible to enhance cycle life because volume change of the silicon phase and pulverization of the negative electrode material, which can occur along with Li intercalation/de-intercalation, are suppressed. Additionally, Patent Document 3 states that, by employing gas atomization method in the fabrication of the above-mentioned alloy particles, it is advantageously possible to fabricate a negative electrode with high charging density, since grinding is not necessary, spherical micropowder can be fabricated, and the negative electrode material in the spherical micropowder form as obtained provide superior chargeability.
Japanese Unexamined Patent Application Publication No. 2007-165300 (Patent Document 4) suggests fabrication of alloy particles containing metal-state Fe and including a phase (A phase) containing at least Si and a phase (B phase) containing an intermetallic compound of at least one type of transition metal element mentioned above and Si, by performing mechanical alloying treatment. According to this suggestion, it is possible to suppress deterioration of storage characteristic due to metal-state Fe dissolving in the negative electrode in the event of storage under overdischarge condition, and additionally, the mechanical synthesis is a way of not only obtaining amorphous or low-crystalline state easily, but also obtaining homogeneous alloy particles.
Indeed, Patent Documents 2 to 4 provide brilliant suggestions for sufficiently retaining high capacity of silicon. However, considering the property of silicon which is hard, but soft at the same time, it needs such a structure that can respond to expansion/contraction in order to completely suppress pulverization.
Further, nonaqueous lithium secondary battery forms solid electrolyte interface (SEI) film on the surface of negative electrode during initial charging and discharging. For example, when graphite is used, it is understood that the SEI film formed by reaction with electrolyte during initial charging becomes stable film which acts to suppress reaction of the second cycle and then on. However, it has yet to be discovered about whether silicon develops SEI film or not. For example, even if SEI film is formed on the surface of silicon or Si-containing active material by the initial charging as in the case of graphite, it is expected that silicon (phase) would break down at least partly due to expansion thereof, and side reaction would occur during the second cycle and every charging thereafter, leaving deposit of the side reaction products or oxidized silicon, and thus resulting in deteriorated charge-discharge cycles.